I have rebuilt my engine and need new pushrods. The block has been decked and the heads are aftermarket and I have been trying to find out the correct push rod lenght. I have comp cams adjustable push rod and used a black felt marker on the valve stem. I have done a search but could not get a real conclusive answer. I know the mark on the stem should not be more than 0.08 inch and the thinner the better. Also, the mark should be as centered as possible on the stem. I am having trouble making out how to get this correct though; If you look in the pic, the upper mark is where the lifter is on the base circle (actually I whiggled the rocker arm to make it more visible, otherwise it is almost invisible)-should this be counted as the starting point OR should the 0.08 apply only to the big (lower) mark where the spring pressure is the highest? I have tried to get the big mark closer to the center but that almost position the rocker roller on the edge of the stem.



The engine is a -90 L98 Hydraulic roller with CC pro magnum rockers.

 

I found that using a dry-erase marker made everything easier to see.

When I did my pushrods I just tightened the rocker down, turned over the motor, and then checked the sweep. There was only one mark.

If that picture is facing the way I think it is I would try a longer rod.

Edit: HERE is my thread on this topic. Most of my issue was me being far to picky about what it looks like and not really knowing what I was doing, but I'm glad I was picky because it runs beautifully now.

 

Go here it will tell

http://www.circletrack.com/techartic...ory_imep_tech/

ya:http://www.compcams.com/information/Products/Pushrods/

 

Thanks Confused and 1racerdude, very good info! I tried a dry-erase marker, a lot better than the one I used. Well, here is the result: It looks like it is close to the stock lenght (7.2) even though the block has been decked and aftermarket heads, I guess these factor negates each other. Topside in the photos points towards the intake side.

7.1 inch looks a little short, the high lift part (2/3 lift) is a however quite close to the center of the stem (but has passed the center and is approaching exhaust side). Mark is approx 0.13 inch wide.



7.15 inch also a little short but pushes the 2/3 lift point slightly more towards exaust side. The mark slightly smaller (0.11 inch).



7.2 inch gets the mark centered but put the 2/3 lift mark slightly more towards exhaust side. The mark about the same size (0.11 inch).


In general, the mark at 2/3 lift is almost the same as for 1/2 lift (no pics though).

I´m leaning towards the 7.15 inch rod since it will put a tad more of the high lift spring pressure closer to the center of the stem than the 7.2-any opinions?

 

It is too wide. Try a couple of different lengths and see. Go at least .050 at a time to find out which way to go.
The width should be .060 or less and centered as best as you can.

 

Well, I did go all the way up to 7.4 and the mark gets smaller but not to 0.06. At 7.4 the mark was approx 0.09. This is how it looks at 7.4:



I guess going further will make the mark smaller but even at 7.4 all roller movement is on the exhaust side of the stem. To reach 0.06 will prob need more than 7.5 lenght. I was under the impression that putting all spring load on one edge will create high side loads on the valve leading to premature failure.

I´ve noticed two different opinions in other threads and I have failed to find a concencus; some seems to prioritize putting the mark on (or near) the centerline, others prefer a small mark with less importance put on location on the stem.

SOmething that might be considered is the fact that the ARP brand rocker studs are 10.9mm in diameter and the rocker holes 11.1mm (so it is possible to slightly wiggle the rockers).

 

OK read this,it's the definative way to do it.

http://www.mid-lift.com/INTRO-ML-BKGRND.htm

 

I managed to read a couple of pages but got lost quite soon. The mid-lift principle was however quite easy to grasp so I decided to try it. This was how I did it:
Firstly I installed a soft check spring and jammed the valve at 50% of maximum valve lift. Next step was to put the rocker arm on the stud and position it so the axis rocker trunnion-roller tip and the valve was at an angle of 90 degrees. I marked the position of the rocker arm at the rocker stud. The check spring was replaced by the regular spring, the head reinstalled on the block and the rocker arm put back on the stud in the marked position. With the cam at base circle I used my adjustable push rod to set zero lash. Removed push rod and measured lenght: 7.65 inch! Quite a difference vs the stock one (7.2).

I used my marker pen an did some more testing with different push rod lenghts:

7.65 gave a mark width of approximately 0.07 inch


7.5, mark around 0.06 inch


7.4, mark around 0.08 inch


7.3, mark around 0.10 inch


It looks like my quite crude setting when trying the miller mid lift principle got quite close. I would probably have done these tests with really long push rods anyway but understanding something about the theory behind valve action feels a lot better than just trail and error tests. The big question now what to choose; a thin/small offset mark OR a wide center mark. The quest goes on...

 

Looking a LOT better.

 

After discussing with three professional engine builders what to prioritize I´ve decided to go for a central mark and put less attention to the swep width. All three recommended a 7.25" rod. To verify this I did a final test and here is the outcome:

Intake


Exhaust


The swep is around 0.10, slightly more than desirable but without changing to more exotic valve train parts I think this is the best I can get.

A big thankyou to all concerned for your input!
/Göran